The present invention generally relates to A/D conversion, and in particular to a high sampling rate A/D conversion method and apparatus.
The development of, for example, telecommunication systems demands A/D conversion at increasingly higher sampling rates. Modern fast electronic A/D converters typically operate at a sampling rate of the order of 50 Mega samples per second, which is much lower than the desired sampling rate of the order of 1 Giga samples per second or higher. Optical solutions for A/D conversion have been suggested to increase the sampling rate. One example is a method involving banks of Mach-Zender interferometers, see [1]. However, the necessary modulators have been considered to be too bulky. Furthermore, this method creates problems with regard to electrical crosstalk between the modulators. Other disadvantages include that the terminations consist of a bank of capacitors connected in parallel and that a pulsed light source is required.
Reference [2] describes an arrangement that converts a voltage into an angle and subsequently converts the angle into a binary pattern. The voltage-angle conversion relies on mechanical, acoustic or electro-optical devices. This severely limits the obtainable conversion rate. Furthermore, the conversion from angle to binary signal is performed by a bulky optical system that is unsuitable for integration.
Another approach has been a complicated arrangement to xe2x80x9ctime stretchxe2x80x9d the analog signal using chirped optical pulsing, see [3].
An object of the present invention is to provide an opto-electronic A/D converting method and apparatus that avoid these problems and are capable of high speed A/D conversion.
This object is achieved in accordance with the attached claims.
Briefly, the present invention involves a tunable laser, the wavelength of which is modulated by the analog signal. The modulated laser beam passes through a grating, which produces a deflected beam. The angle of deflection corresponds to the amplitude of the analog signal. The deflected beam impinges on a specific kinoform in a kinoform array. The impinged kinoform produces a corresponding bundle of beams that is directed to an array of photo detectors. Each kinoform in the array produces a different bundle of beams, and each bundle corresponds to a different digital value. The power distribution on the array of photo detectors is sampled to determine the digital value.
The described arrangement has several advantages:
1. It is possible to achieve A/D conversion at very high sampling rates, more than 1 Giga samples per second for a resolution of 6-8 bits.
2. Several vital elements used for the actual A/D conversion (grating and kinoforms) are stable passive elements that are not sensitive to sampling frequency.
3. The A/D converter itself has a low power consumption (about 10 mW for the laser and 10 mW per digital bit).
4. The actual A/D converter is small, typically less than 20xc3x974xc3x971 mm.